Biological evolution refers to the cumulative changes that occur in a population over time. These changes are produced at the genetic level as organisms’ genes mutate and/or recombine in different ways during reproduction and are passed on to future generations. Sometimes, individuals inherit new characteristics that give them a survival and reproductive advantage in their local environments; these characteristics tend to increase in frequency in the population, while those that are disadvantageous decrease in frequency. This process of differential survival and reproduction is known as natural selection. Non-genetic changes that occur during an organism’s life span, such as increases in muscle mass due to exercise and diet, cannot be passed on to the next generation and are not examples of evolution.
Darwin‘s theory of evolution by natural selection was considered dangerous in 19th-century England because it threatened the prevailing views of the Anglican Church and society at large. If extrapolated beyond its role as a scientific theory, it seemed to run counter to the notion of humankind’s supremacy and God’s hand in the world order. The same fear exists today. Evolution can seem to be in direct conflict with deeply held belief systems of how life came to exist on Earth.
2. Isn’t evolution just a theory that remains unproven?
In science, a theory is a rigorously tested statement of general principles that explains observable and recorded aspects of the world. A scientific theory therefore describes a higher level of understanding that ties “facts” together. A scientific theory stands until proven wrong — it is never proven correct. The Darwinian theory of evolution has withstood the test of time and thousands of scientific experiments; nothing has disproved it since Darwin first proposed it more than 150 years ago. Indeed, many scientific advances, in a range of scientific disciplines including physics, geology, chemistry, and molecular biology, have supported, refined, and expanded evolutionary theory far beyond anything Darwin could have imagined.
3. Are all species related?
Yes. Just as the tree of life illustrates, all organisms, both living and extinct, are related. Every branch of the tree represents a species, and every fork separating one species from another represents the common ancestor shared by these species. While the tree’s countless forks and far-reaching branches clearly show that relatedness among species varies greatly, it is also easy to see that every pair of species share a common ancestor from some point in evolutionary history. For example, scientists estimate that the common ancestor shared by humans and chimpanzees lived some 5 to 8 million years ago. Humans and bacteria obviously share a much more distant common ancestor, but our relationship to these single-celled organisms is no less real. Indeed, DNA analyses show that although humans share far more genetic material with our fellow primates than we do with single-celled organisms, we still have more than 200 genes in common with bacteria.
It is important to realize that describing organisms as relatives does not mean that one of those organisms is an ancestor of the other, or, for that matter, that any living species is the ancestor of any other living species. A person may be related to blood relatives, such as cousins, aunts, and uncles, because she shares with them one or more common ancestors, such as a grandparent, or great-grandparent. But those cousins, aunts, and uncles are not her ancestors. In the same way, humans and other living primates are related, but none of these living relatives is a human ancestor.
4. What is a species?
Members of one species do not normally interbreed with members of other species in nature. Sometimes, members of different species, such as lions and tigers, can interbreed if kept together in captivity. But in nature, geographic isolation and differences in behavior, such as choice of habitat, keep these sorts of closely related animal species apart. Similarly, closely related species of plants can sometimes be hybridized by horticulturists, but these hybrids are rarely found in nature. A species, then, is defined by science as a group of interbreeding or potentially interbreeding populations that is reproductively isolated from other such groups.
5. What do genes have to do with evolution?
Genes are the portions of an organism’s DNA that carry the code responsible for building that organism in a very specific way. Genes — and, thus, the traits they code for — are passed from parent to offspring. From generation to generation, well-understood molecular mechanisms reshuffle, duplicate, and alter genes in a way that produces genetic variation. This variation is the raw material for evolution.
6. What role does sex play in evolution?
Sexual reproduction allows an organism to combine half of its genes with half of another individual’s genes, which means new combinations of genes are produced every generation. In addition, when eggs and sperm are produced, genetic material is shuffled and recombined in ways that produce new combinations of genes. Sexual reproduction thus increases genetic variation, which increases the raw material on which natural selection operates. Genetic variation within a species — also known as genetic diversity — increases a species’ opportunity for change over successive generations.
7. Is evolution a random process?
Evolution is not a random process. The genetic variation on which natural selection acts may occur randomly, but natural selection itself is not random at all. The survival and reproductive success of an individual is directly related to the ways its inherited traits function in the context of its local environment. Whether or not an individual survives and reproduces depends on whether it has genes that produce traits that are well adapted to its environment.
8. Are evolution and “survival of the fittest” the same thing?
Evolution and “survival of the fittest” are not the same thing. Evolution refers to the cumulative changes in a population or species through time. “Survival of the fittest” is a popular term that refers to the process of natural selection, a mechanism that drives evolutionary change. Natural selection works by giving individuals who are better adapted to a given set of environmental conditions an advantage over those that are not as well adapted. Survival of the fittest usually makes one think of the biggest, strongest, or smartest individuals being the winners, but in a biological sense, evolutionary fitness refers to the ability to survive and reproduce in a particular environment. Popular interpretations of “survival of the fittest” typically ignore the importance of both reproduction and cooperation. To survive but not pass on one’s genes to the next generation is to be biologically unfit. And many organisms are the “fittest” because they cooperate with other organisms, rather than competing with them.
9. How does natural selection work?
In the process of natural selection, individuals in a population who are well-adapted to a particular set of environmental conditions have an advantage over those who are not so well adapted. The advantage comes in the form of survival and reproductive success. For example, those individuals who are better able to find and use a food resource will, on average, live longer and produce more offspring than those who are less successful at finding food. Inherited traits that increase individuals’ fitness are then passed to their offspring, thus giving the offspring the same advantages.
10. How do organisms evolve?
Individual organisms don’t evolve. Populations evolve. Because individuals in a population vary, some in the population are better able to survive and reproduce given a particular set of environmental conditions. These individuals generally survive and produce more offspring, thus passing their advantageous traits on to the next generation. Over time, the population changes.
11. Does evolution prove there is no God?
No. Many people, from evolutionary biologists to important religious figures like Pope John Paul II, contend that the time-tested theory of evolution does not refute the presence of God. They acknowledge that evolution is the description of a process that governs the development of life on Earth. Like other scientific theories, including Copernican theory, atomic theory, and the germ theory of disease, evolution deals only with objects, events, and processes in the material world. Science has nothing to say one way or the other about the existence of God or about people’s spiritual beliefs.